1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device comprising a bipolar integrated circuit and, more particularly to a method for manufacturing a semiconductor device using silicon microcrystal as an emitter.
2. Description of the Related Art
Conventionally, as one of the heterojunction bipolar transistor which can ensure a sufficient amplification factor even at the temperature of liquid nitrogen (77K), there has been proposed a technique in which a silicon microcrystal is used as an emitter. For example, as is disclosed in the article by Fujoika et. al. titled "Low Temperature operation of a Si HBT" in proceedings of "Conference on Solid State Devices and Materials", pp. 125-128 (1988), by the use of silicon microcrystal layers deposited by a plasma chemical vapor deposition method, there is provided a ring oscillator of an ECL comprising a heterojunction bipolar transistor.
However, in this type of the semiconductor device, the following problems occurred.
Specifically, when this semiconductor device is operated at the temperature of liquid nitrogen (77K), gate delay time becomes relatively large, e.g. 295 (ps). One of reasons why the gate delay time becomes large is that a connected resistance between an aluminum electrode and a silicon microcrystal layer is increased. Since the temperature is low such as 250.degree. C. to 450.degree. C. when the microcrystals are deposited, heat treatment after the deposition of the aluminum electrode cannot be sufficiently performed. If the heat treatment is performed at a high temperature to reduce the connected resistance, the quality of the silicon microcrystal is changed and its band gap is narrowed. As a result, the heterojunction between the emitter and base, which is a feature of the structure of this type of transistor, is lost.
As mentioned above, in the conventional heterojunction bipolar transistor using a silicon microcrystal layer as an emitter, if the temperature of the heat treatment after the deposition of the silicon microcrystals is low, the connected resistance between the aluminum electrode and the silicon microcrystal layer is increased. Oppositely, if the temperature of the heat treatment is high, the quality of the silicon microcrystal is changed, thereby heterojunction is lost.